Habitable Planet Reality Check: Kepler 186f Revisited

One of the objectives of NASA’s Kepler mission launched on March 7, 2009, as well as one of the important drivers of its design, was to detect the transits of Earth-size planets in Earth-like orbits around Sun-like stars – the sorts of worlds that one could expect to be potentially habitable and abodes for life as we know it. But from the start, it was known that several years of data would be required to observe a minimum of three transits from such extrasolar planets along with follow on observations from the ground to confirm such finds. As an additional complication, it was discovered during the analysis of Kepler data that most Sun-like stars were photometrically more noisy than the Sun. Combined with other issues, the detection of Earth-size planets in Earth-like orbits around Sun-like stars (i.e. true Earth twins) has turned out to be more involved and taking longer than originally hoped.

Diagram showing the major components of NASA’s Kepler spacecraft. (NASA/Kepler Mission/Ball Aerospace)

But as the analysis of data from Kepler’s already completed primary mission continues and new data from the extended “K2” mission flows in, there has been a flood of thousands of confirmed extrasolar planets and candidates that are easier to detect: exoplanets in small orbits with short periods as well as planets larger than the Earth especially those associated with stars smaller than the Sun. While there had been a handful of exoplanet discovery announcements which had made rather dubious claims of being potentially habitable, on April 17, 2014 a group of scientists working on NASA’s Kepler mission announced the discovery of what was billed as the most Earth-like planet found to date. Designated Kepler 186f, this extrasolar planet was an Earth-size world orbiting inside the habitable zone (HZ) of a star smaller than the Sun. While hardly the “Earth twin” we had all been waiting for, it seemed to have the potential of at least being “Earth-like” in many important ways.

A diagram showing the location of extrasolar planets known in early 2014 in relation to various definitions of the habitable zone as a function of effective stellar flux and stellar temperature. The size of the planet symbols is proportional to its measured or inferred mass. Kepler 186f in highlighted in the lower right corner. Click on image to enlarge. (NASA/Chester Harman)

While the discovery of Kepler 186f was surrounded by much media hype at the time, like many earlier announcements of exoplanets which were claimed to be potentially habitable, an objective assessment of this new find did reveal it had reasonable prospects of being habitable (see “Habitable Planet Reality Check: Kepler 186f”). But with much more data in hand from the rest of the Kepler mission along with new follow up observations, how does the claim of the potential habitability of Kepler 186f hold up a couple of years after its discovery?

Background

The red dwarf star called Kepler 186 (also known as KIC 8120608) is located in the constellation of Cygnus, the Swan. The best data currently available from the NASA Exoplanet Archive indicates that this type M1V star is at an estimated distance of 561 +42/-33 light years with an apparent V-magnitude of only about 15.6. The best fit results for this star’s properties indicates that it has a radius 0.52 times that of the Sun, a mass of 0.54 times and a luminosity of 0.055 times – all in all Kepler 186 is a fairly large red dwarf, as these diminutive stars go.

The arrow indicates the position of the red dwarf Kepler 186 in this K-band image from the 2MASS all-sky survey. (NASA)

The discovery of the first four planets orbiting Kepler 186, designated b through e, was first discussed in a number of forums starting in August of 2013 and was officially announced in February 2014. Details on the confirmation of these finds based on the first two years of Kepler data were presented in a pair of papers by Lissauer et al. and Rowe et al. published in the March 20, 2014 issue of The Astrophysical Journal. With the benefit of an additional year’s worth of data, a fifth planet known as Kepler 186f was confirmed and its discovery officially announced in a press conference on April 17, 2014 with the details following in a paper by Quintana et al. published in the April 18 issue of Science.

What was notable about this discovery was that Kepler 186f had a radius initially estimated to be 1.11 ±0.14 times that of the Earth (or R_E) with an orbital period of 129.9 days resulting in a effective stellar flux, S_eff, of 0.32 times that of the Earth. Kepler 186f was clearly an Earth-size planet that appeared to be orbiting comfortably inside the HZ of the star it orbited. Unlike most earlier claims made about previously discovered exoplanets which were too large to be rocky planets and frequently stretched the definition of the HZ to sometimes absurd extremes, Kepler 186f was clearly a potentially habitable planet.

Diagram showing the relative sizes of the Kepler 186 and our inner Solar System. The gray ring surround each star indicates a more liberal definition of the habitable zones (HZ) around each star. Click on image to enlarge. (NASA Ames/SETI Institute/JPL-Caltech)

Analysis of a full four years of Kepler data coupled with follow up observations made from the ground allowed for a refinement of the properties of Kepler 186f and the red dwarf it orbits. These results were formally published in February 2015 in a paper by Torres et al. which also included refinements in the properties of some other early Kepler finds as well as the presentation of eight newly discovered HZ planets (for a full discussion of this work, see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”). The radius of Kepler 186f was now pegged at 1.17±0.08 R_E with a mean orbital radius of 0.432 +0.171/-0.053 AU and a S_eff of 0.30 +0.10/-0.15. The properties of the known planets in the Kepler 186 system based on the best data currently available are presented in the table below.

Properties of Planets Orbiting Kepler 186

Potential Habitability

A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets is basic orbit parameters, a rough measure of its size or mass and some important properties of its sun. Combined from theoretical extrapolations of the factors that keep the Earth habitable (not to mention why our neighbors Venus and Mars are not), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean habitable in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface. While there may be other worlds that might possess environments that could support life (e.g. Mars or the tidally heated oceans on the moons Europa and Enceladus), these would not be Earth-like habitable worlds of the sort being considered here.

One of the important properties of an extrasolar planet that can be used indirectly to assess its potential habitability is the orbital period. Combined with a knowledge of the target star’s properties, the size of the planet’s orbit and its effective stellar flux, S_eff, can be determined. According to the work by Kopparapu et al. on the limits of the HZ based on detailed climate modeling, the outer limit of the HZ is conservatively defined as corresponding to the maximum greenhouse limit of a CO₂-rich atmosphere where the addition of any more CO₂ would not increase a planet’s surface temperature any further. For a star like Kepler 186 with a temperature of 3755 K, this conservative outer limit for the HZ has an S_eff of 0.26 corresponding to a distance of 0.46 AU. Based on the work of Torres et al., the S_eff for Kepler 186f is 0.30 +0.10/-0.15 which lies comfortably inside this outer limit. Given the uncertainties in the properties of Kepler 186f and the star it orbits, Torres et al. estimate that there is a 98.4% chance that Kepler 186f orbits inside the HZ. The other four planets known to be orbiting Kepler 186 have S_eff values that are too high to be inside the HZ by any reasonable definition.

Diagram the planets of our Solar System and Kepler 186 showing a more liberal definition of the HZ and how it changes with increasing star temperature. Click on image to enlarge. (NASA)

The next vital piece of information that can be derived using the transit method employed by the Kepler mission is the radius of the planet. But this alone is insufficient to constrain the bulk composition of a planet – it could be a rocky planet like the Earth or a volatile-rich mini-Neptune with no chance of being habitable in the conventional sense. With a radius of 1.17 R_E, Kepler 186f would have a mass of about 1.8 times that of the Earth (or M_E) if it had an Earth-like composition – less if it were more Neptune-like. Unfortunately, the radial velocity variation of about 0.3 meters per second expected with even with an Earth-like composition is not detectable by instruments employed in radial velocity surveys and, given the dimness of Kepler 186, even the next generation of instruments currently planned. Likewise, Kepler 186 itself is too dim for there to be any prospect of detecting the atmosphere of Kepler 186f using the James Web Space Telescope or other instrument being planned. Unless the mass of Kepler 186f can be determined indirectly by other methods like Transit Timing Variations (TTV), it is unlikely that its bulk composition can be determined anytime soon.

In lieu of a mass value to constrain the bulk composition of Kepler 186f, we do have statistical arguments based on the observed properties of other extrasolar planets – a source of information that was only just becoming available in early 2014. An analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Leslie Rogers (Hubble Fellow at Caltech) and formally published a year after the discovery of Kepler 186f strongly suggests that planets transition from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 R_E (for a detailed description of this work, see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). With a radius of 1.17 R_E, Kepler 186f is comfortably below this 1.6 R_E threshold suggesting that it is more likely to be a terrestrial planet.

Since the publication of Rogers’ work, others have attempted a more quantitative approach to determine the probability that various sized exoplanets have a rocky composition. Using a method that gave qualitatively similar results as Rogers’, Torres et al. estimated that there is a 68.4% probability that Kepler 186f is a rocky planet. A more recent analysis of the mass-radius relationship with a much larger collection of exoplanetary data by Jingjing Chen and David Kipping (Columbia University) suggests that Kepler 186f has a mass of 1.74 +1.31/-0.60 M_E, roughly in keeping with an Earth-like composition (see “The Composition of Super Earths”). Based on a statistical analysis of known exoplanetary radii and masses as well as the current uncertainties in the properties of Kepler 186f, they estimate that Kepler 186f has a 59% probability of being a rocky planet like the Earth.

An artist’s depiction of Kepler 186f, shown as an Earth-like world, compared to the Earth. (PHL)

So it appears that Kepler 186f orbits inside the HZ and, based on its radius, it is most likely a rocky planet. But what would the conditions on the planet be like? A paper by Bolmont et al. published only five months after the announcement of the discovery of Kepler 186f addressed this question in detail. Bolmont et al. found that the Kepler 186 system is dynamically stable and that there has been insufficient time for the orbits of Kepler 186c through f to have evolved significantly since their formation due to tidal effects . Therefore, these outer four planets most likely formed where they are found today.

The spin states of these worlds as a result of tidal interactions with Kepler 186 is a different matter. Bolmont et al. found that the inner four planets would have almost certainly slowed to become synchronous rotators or, depending on their orbital eccentricities (which are poorly constrained by the Kepler transit data), enter a super-synchronous state where their period of rotation is a small integer fraction of their orbital period. Mercury, which rotates three times for every two orbits around the Sun, is an example of a planet with such a super-synchronous rotation state. Bolmont et al. were more uncertain about Kepler 186f given that the age of the Kepler 186 system was not known at the time of their work as well as the lack of detailed information on the initial spin state and tidal characteristics of Kepler 186f. Since then, the best estimate of the system’s age by Torres et al. has been found to be 4.0±0.6 billion years – just a bit younger than our solar system. At such an age, it is almost certain that Kepler 186f is a synchronous or super-synchronous rotator regardless of its initial spin state or internal structure. An ever increasing body of scientific literature from the past two decades has shown that such a rotation state is not an impediment to planetary habitability as was once widely believed.

While work by Kopparapu et al. and others indicate that Kepler 186f is safely within the HZ, Bolmont et al. performed their own simulations to verify this and gain additional insights into the surface conditions on Kepler 186f. They used a simple 1D, cloud-free radiative-convective model with an atmosphere dominated by N₂, CO₂ and H₂O with a volatile budget comparable to that of the Earth (as well as the initial volatile inventories of Venus and Mars, before they evolved towards their current states). Bolmont et al. found that surface temperatures could be kept above freezing with CO₂ concentrations between 0.5 to 5 bars for N₂ partial pressures ranging from 10 to 0 bars, respectively. If Kepler 186f is a rocky planet with a roughly Earth-like inventory of volatiles, it seems that it could be habitable if it has sufficient levels of geologic activity to maintain the carbonate-silicate cycle to act as a planetary thermostat and if there are no other impediments to planetary habitability associated with being located in an M-dwarf system.

The final state of 20 simulated systems compared with Kepler 186 (middle) suggests that one or two additional planets may orbit between Kepler 186e and f. The size of each simulated planet was scaled assuming a rocky composition. Click on image to enlarge. (Bolmont et al.)

Another interesting finding by Bolmont et al. is that Kepler 186f is most likely not alone in the HZ. Their simulations of the formation of the Kepler 186 system show that one or two additional planets about the size of the Earth should have formed between the orbits of Kepler 186e and f that would still be within the HZ. Dynamical calculations showed that such an arrangement would be stable and easily avoid creating transits detectable from the Earth. Subsequent statistical work by Ballard and Johnson on the architecture of multi-planet systems associated with red dwarfs like Kepler 186 show that such systems should be expected to have, on average, 7.5 +0.5/-1.5 planets with low mutual inclination of 2.0±1.3° (for a description of the earlier findings by Ballard and Johnson, see “Architecture of M-Dwarf Planetary Systems”). Work by Dressing and Charbonneau shows that Earth-size planets are common orbiting red dwarfs while Neptune-size worlds and larger are exceedingly rare (see “The Occurrence of Potentially Habitable Planets Around Red Dwarfs”). With transits from only five planets detected, Kepler 186 could easily have a couple of more, possibly Earth-size, planets that might also be potentially habitable like Kepler 186f.

Conclusions

Even two years after the announcement of the discovery of Kepler 186f, it still seems to be a reasonable candidate for being potentially habitable – it orbits comfortably in the outer part of the habitable zone of its sun and, given its measured radius, it seems likely that this extrasolar planet has a rocky composition when compared to other planets of similar size. If it has an volatile inventory similar to Earth’s, it would be expected to have above-freezing temperatures on its surface with a modestly dense atmosphere with a few bars of CO₂. This atmosphere, combined with an ocean that could be present, could easily maintain habitable conditions anywhere on this world even if it is a synchronous or slow rotator as seems likely.

While there are a number of unresolved issues with the potential habitability of any planets orbiting red dwarfs, Kepler 186 is a rather large size star for its class which lessens the impact of these issues especially given the large size of the orbit of Kepler 186f. Even with these issues in mind, Kepler 186f remains one of the best candidates for being potentially habitable of all currently known extrasolar planets along with Kepler 62f and Kepler 442b (see “A Review of the Best Habitable Planet Candidates”). And there is also the distinct possibility that Kepler 186f is accompanied by at least one undetected sister planet also in the habitable zone.

An artists depiction of the various small planets located in the habitable zone discovered by NASA’s Kepler mission. Click on image to enlarge. (NASA)

While it seems that Kepler 186f is a good candidate for being a potentially habitable planet, unfortunately the prospects of learning much more about this world in the near future are not very promising. The relative dimness of Kepler 186 makes it a poor target for follow up observations using the current or even the next generation of astronomical instruments. NASA’s TESS and ESA’s CHEOPS missions will continue Kepler’s search for transiting planets but they will be focused on stars much brighter than Kepler 186 and will not provide new data on this system. It could be some time before new significant data on Kepler 186f is obtained as the focus of scientists turns to more easily observed targets. Despite this, Kepler 186f has at least demonstrated that potentially habitable planets do exist pointing the way towards more Earth-like finds to come.

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Related Reading

“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]

“Habitable Planet Reality Check: 8 New Habitable Zone Planets”, Drew Ex Machina, January 8, 2015 [Post]

“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]

“Architecture of M-Dwarf Planetary Systems”, Drew Ex Machina, October 24, 2014 [Post]

“The Occurrence of Potentially Habitable Planets Around Red Dwarfs”, Drew Ex Machina, January 12, 2015 [Post]

General References

Sarah Ballard and John Asher Johnson, “The Kepler Dichotomy among the M Dwarfs: Half of Systems Contain Five or More Coplanar Planets”, The Astrophysical Journal, Vol. 816, No. 2, Article id. 66, January 8, 2016

Emeline Bolmont et al., “Formation, Tidal Evolution, and Habitability of the Kepler-186 System”, The Astrophysical Journal, Vol. 793, No. 1, Article id. 3, September 20, 2014

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, arXiv 1603.08614, March 29, 2016 [Preprint]

Courtney D. Dressing and David Charbonneau, “The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity”, The Astrophysical Journal, Vol. 807, No. 1, Article id. 45, June 30, 2015

R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

J. J. Lissauer et al., “Validation of Kepler’s multiple planet candidates. II: Refined statistical framework and systems of special interest”, The Astrophysical Journal, Vol. 784, No. 1, Article ID. 44, March 20, 2014

Elisa V. Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star”, Science, pp. 277-280, Vol. 344, No. 6181, April 18, 2014

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

J.F. Rowe et al., “Validation of Kepler’s multiple planet candidates. III: Light curve analysis & announcement of hundreds of new multi-planet systems”, The Astrophysical Journal, Vol. 784, No. 1, Article ID. 45, March 20, 2014

Guillermo Torres et al., “Validation of 12 Small Kepler Transiting Planets in the Habitable Zone”, The Astrophysical Journal, Vol. 800, No. 2, Article id. 99, February 20, 2015

Kepler 186f, NASA Exoplanet Archive, NASA Exoplanet Science Institute, retrieved April 17, 2016 [Web Page]